Biological membranes + Cell DDD Flashcards
What is the function of a cell memebrane
membranes form barriers and separate the cell contents from the cells exterior environment. Or separate organelles from the cytoplasm, they need to allow some molecules through, into or out of the cell. So some membranes also have permeability.
What is permeability
Permeability refers to the ability to let substances pass through. Some very small molecules simply diffuse through the cell membrane, in its structural molecules. Some substances dissolve in the lipid layer and simply pass through. And finally some substances pass through through special protein channels or carriers.
What type of permeability do plasma cell membranes have and why ?
Partially permeable- only allow some substances through. The properties of the membranes components determines its permeability. For example how many proteins it contains, ect.
What is the role of membranes at the surface of cells
The plasma membrane: separates the cells components from its external environment (in single celled organisms the environment is its external environment) the membrane regulates the transport of materials in and out of the cell. It may also contain enzymes involved in many metabolic pathways. They also have antigens so that the organisms immune system can recognise the cell as self and not attack it. Finally, membranes may release chemicals to other cells.
What is the role of membranes within cells
The membranes found around organelles in eukaryotic organisms separate the cell contents from the cytoplasm so each organelle is its own discrete entity. For example, mitochondria have a folded inner membrane called cristae, this gives large surface areas for the reactions of aerobic respiration.
As well as this, chloroplasts have inner membranes called thylakoids, which house chlorophyll. On these membranes stages of photosynthesis occurs.
State the fluid mosaic model of cell membrane structure
In 1972, singer and Nicholson proposed a model that allowed the passage of molecules through the membrane. Their structure explained how membranes could be more dynamic and interact more with the cells environment. It was called the fluid mosaic model. And proposed that the fabric of the membrane consisted of a phospholipid bi-layer. With proteins embedded floating within it, making up a mosaic pattern. The lipid molecules can change places with each other, and some proteins move giving it more fluidity. The fabric of the membrane, made out of the lipid bilayer, have their hydrophilic polar heads on contact with the water exterior and interior of the cell. The hydrophobic tails are then tucked away inside the membrane.
State and Describe the various proteins involved within membranes
Some of the proteins have pores and act as channels to allow ions, which have an electrical charge and are surrounded by water molecules, to pass through.
Some of the proteins are carriers and, by changing their shape, carry specific molecules across the membrane.
And finally, other proteins may be attached to the carrier proteins and function as enzymes, antigens or receptor sites for complementary-shaped signalling molecules such as hormones.
What is the function of cholesterol in membranes
The function of cholesterol in the cell membrane is to stabilize the interactions between the phospholipids and prevent the membrane from getting too fluid or too flexible. It also resists temperature changes on the structure of the membrane.
What is the total thickness of the cell membrane
Between 5 and 10 nm
What is the glycocalyx
The glycocalyx is outside the membrane of cells and is formed by carbohydrate chains attached to either lipids (glycolipids) or proteins (glycoprotein) in the membrane.
Describe the different compositions of cell membranes in neurones, white blood cells and root hair cells
In neurone, the protein channels and carriers in the plasma membrane covering the long axon allow entry and exit of ions to bring about the conduction of electrical impulses along their length. As well as this they have a myelin sheath formed by flattened cells wrapped around them several times, giving several layers of cell membrane. This is about 20 percent proteins and 76 percent lipids, making the axon incredibly insulated.
The plasma membrane of white blood cells contain special protein receptors that enable them to recognise the antigens on foreign cells, usually from invading pathogens but also from tissue or organ transplants.
Root hair cells in plants have many carrier proteins to actively transport nitrate ions from the soil and into cells
The inner membranes of mitochondria are 76 % protein and 24 % lipid. This is because their inner membrane contain many electron carriers that are made of protein, and hydrogen ion channels associated with ATP synthase enzymes.
What is simple diffusion and describe its process
All molecules have kinetic energy and can move freely and randomly within gas or liquid media. This will happen even if the medium is not mixed by stirring or shaking. If there is a higher concentration of a certain type of molecule in an area, then the molecules will bump into eachother as they randomly move, and eventually they will spread further from each-other. More will move to an area of low concentration, until eventually they become evenly dispersed. They reach equilibrium as there is no net movement. Some molecules which are small and non polar can pass through membranes by this process of simple diffusion. A-swell as this, fat-soluble molecules such as steroid hormones, even if they are larger, can diffuse through as they dissolve in the lipid bilayer.
How do water molecules diffuse across plasma membranes
Water molecules are a special case. Since they are polar and insoluble in the lipid bi layer it would seem impenetrable. However water, is present in such great concentrations that significant direct diffusion does happen. In membranes where a very high rate of water molecules are required there may be specific water channel proteins known as aquaporins. These allow water across the membranes without moving through lipid environment.
How is concentration maintained and give two examples
Many molecules entering cells then pass into organelles are used for metabolic reactions: this maintains a concentration gradient and keeps more molecules entering cells. For example. Oxygen diffusing into the cytoplasm of respiring cells then diffuses into mitochondria and is used for aerobic respiration.
As well as this carbon dioxide diffusing into palisade mesophyll cells of plants will then diffuse into chloroplasts and can be used for photosynthesis.
What factors effect the rate of simple diffusion
Simple diffusion relies only on the molecules’ own kinetic energy. And so factors that alter thus kinetic energy will effect this rate of diffusion. For example temperature. As temp increases, molecules have more kinetic energy so their rate if diffusion will increase. Conversely, if they loose heat rate of diffusion slows down.
-diffusion distance, the thicker the membrane across which molecules have to diffuse, the slower the diffusion rate
-surface area, more diffusion can take place across a larger surface area. Cells specialised for absorbsion have extensions to increase their surface area. For example microvilli.
-size of diffusion molecule- smaller ions or molecules diffuse more rapidly than larger molecules
-concentration gradient- the steeper this gradient the faster the diffusion to the side with fewer molecules.
What is facilitated diffusion
Facilitated diffusion is a type of diffusion in which the molecules move from the region of higher concentration to the region of lower concentration assisted by a carrier. It occurs when small molecules have polarity, such as ions with an electrical charge so they cannot pass through the lipid bilayer due to the hydrophilic and hydrophobic tails and heads.
What protein support diffusion do neurone have
Neurone plasma membranes have many channels specific to either sodium or potassium ions. The diffusion of these ions into and out of the neuron axon is crucial to the conduction of nerve impulses. At synapses, there are also calcium ion channels.
What protein support diffusion do epithelial cells have
The plasma membranes of epithelial cels that line your airways have chloride ion channels, and these play a crutial role in regulating the composition of mucus to trap particles and pathogens.
What is the function of the transport protein: channel protein
They create hydrophilic channels to transport polar molecules and ions that cannot pass through the hydrophobic tails of the phospholipid bilayer. It is a type of facillitated diffusion and it is never an active process.
What is the function of the transport protein: carrier proteins
Specific to particular ions or molecules that are too large to fit through phospholipids. This is a type of facilitated diffusion that is active, as it uses ATP to help with it conformational change.
What is the term for when a molecule has both hydrophobic and hydrophilic areas
Amphipathic
What is the function of glycolipids
Glycolipids are carbohydrates attached to the hydrophilic phosphate group head. It helps in cell signalling and recognition.
Define the term osmosis
Osmosis is the net movement of water molecules from an area of high water potential to low water potential across a partially permuable membrane.
What is water potential
Water potential is the measurement of the tendency of water molecules to diffuse from one region to another
What happens when solute molecules are added to water
When solute molecules are added to water, the relative number of water molecules, in the resulting solution, is changed. If the solute molecules dissociate into charged ions such as sodium chloride dissociating into sodium and chloride ions, they exert more effect on the relative number of water molecules than do larger but non polar molecules such as glucose. This is because, the number of particles in the solution doubles and water molecules are attracted to them and prevent them from crossing the membrane because they become to large of molecules.
What is the water potential of pure water
O kPa
What happens to water potential as solute molecules are added
When solute molecules are added, they lower the pater potential of the solution. The more solute molecules in the solution, the lower the water potential.
What are the units for measuring water potential
Water potential is measured in kilopascal (kPa) pure water has the highest potential possible of 0kPa.
What happens when a cell is placed into an area of higher water potential.
The water potential inside cells is lower than that of pure water, as there are solutes in solution, in the cytoplasm and inside the large vacuole of plant cells. When the cells are placed in a solution of higher water potential, then water molecules move by osmosis down the water potential gradient,across the plasma membrane, into the cell.
What is the term for when in an animal cell, a lot of water enter the cell. It will then swell and burst as the plasma membrane breaks.
Cytolysis
What is the term for a plant cell when the cell swells up to a certain size, but the cell wall prevents the cell bursting
Turgid
What is the term for when an animal cell has lost water due to water potential outside the cell being higher than inside the cell, and it becomes shrivelled
Crenated
What is the term for when a plant cells cell membrane shrinks and it pulls away from the cell wall.
Plasmolysed (plant tissue with plasmolysed cells is described as flaccid)
during active transport, where do cells get energy from to carry this out
The energy is provided from the hydrolysis of ATP
Why do carrier proteins carry out active processes of diffusion
These membrane proteins have specific regions, or sites that combine reversible with only certain molecules or ions. They also have a region that binds to and allows the hydrolysis of a molecule of ATP, to release energy, and in this way they act as enzymes. The energy helps the carrier proteins change its conformation, and in doing so it carries the ion from one side of the membrane to the other. For example in guard cells, ATP made by chloroplasts provides energy to actively transport potassium ions from surroundings into the Gard cells. This influx of ions lowered the water potential in the cells so that water enters by osmosis. As the guard cell swells, their tips bulge which opens the stomata.
What is bulk transport
Some cells need to transport large molecules and particles too large to pass through plasma membrane, in or out. They do this by bulk transport, a process that requires energy from ATP. The energy released from the hydrolysis of ATP enables the sodium potassium pump protein to change its shape so that 3 sodium ions are now on the outside of the cell and 2 potassium ions are on the inside of the cell.
What is meant by the term, endocytosis
This is how large particles may be brought into a cell. They do not pass through the plasma membrane. Instead, a segment of the plasma membrane surrounds and encloses the particle and brins it into the cell, enclosed inside a vesicle. For example in phagocytosis. However, ATP is needed to provide energy to form the vesicles and move them, using molecular motor proteins, along cytoskeleton threads into the cells exterior.
What is meant by the term, exocytosis
This is how large molecules may be exported out of the cells. They do not pass through the plasma membrane. Instead a vesicles containing them is moved towards and then fuses with the plasma membrane. An example of this is seen at synapses. Where chemicals in vesicles are moved by motor proteins along cytoskeleton threads, to the presynaptic membrane, here the vesicles fuse with the membrane and the neurotransmitter chemicals are released into the synaptic cleft. In this case. ATP is needed to fuse the membranes together as well for moving the vesicles. Molecule of ATP is needed for every step motor proteins take along cytoskeltal threads.
What happens in the case of phospholipids when loosing temperature (Bi-layer)
-saturated fatty acids become compressed
-however,there are many unsaturated fatty acids making up the cell membrane. And as they become compressed, the kinks in their tails push the adjacent phospholipid molecules away. This maintains membrane fluidity.
-therefore, the proportions of unsaturated and saturated fatty acids within a cells membrane, determine the membranes fluidity at colder temperatures.
-cholesterol in the membrane also buffers the effect of lowered temperature, to prevent a reduction in the membranes fluidity. It does this by preventing the phospholipid molecules from packing together too closely, because cholesterol molecules are between groups of phospholipid molecules.
What happens in the case of phospholipids when temperature increases (bi-layer)
-the phospholipids acquire more kinetic energy and move around, in a more random way. This increases membrane fluidity, aswell as permeability.
-it also affects the way membrane-embedded proteins are positioned and may function. If some of the proteins that act as enzymes in a membrane drift sideways, this could alter the rate of reaction they catalyse.
-the presence of cholestorol molecules buffers the effects of increasing heat and reduces the increase of membrane fluidity.
- an increase in membrane fluidity may effect the plasma membrane during phagocytosis.
-the increase in membrane fluidity may also change the ability of cells to signal each other by releasing chemicals, often by exocytosis.
What happens if membrane structures are distrusted buy high tempratures above proteins optimum temperature
Proteins are not as stable as lipids. So high temperatures cause the atoms within there large molecules to vibrate, and this breaks their hydrogen bonds and ionic bonds that hold their structure, causing the proteins to unfold. Their tertiary structure changes and they cannot change back again as they are denatured. As well as this just under the plasma membrane are cytoskeleton threads made of protein. If both types of proteins are denatured, then the membrane will begin to fall apart and it will become very permeable.
What are the three key stages of the cell cycle
Interphase
Nuclear division (mitosis or meiosis)
Cytokinesis
Describe the processes involved with interphase stage: G1
G1- proteins synthesis occurs to make proteins involved in synthesising organelles
-the organelles replicate
-cell is checked that it is the correct size, has the correct nutrients,growth factors ad that there i no damaged DNA. If check is not passed, replication doesn’t continue.
Describe the processes involved with the interphase stage: S
-DNA is replicated
Once the cell has entered this stage it is committed to finishing the cycle.
